Enhancing the nonlinearity of an optical waveguide

ABSTRACT

An optical device includes a waveguide and a film or layer of a material having highly nonlinear optical characteristics, in particular a semiconductor material. The film is located so close to the waveguide core that the evanescent field of light propagating along the waveguide extends up to the film. In this optical device, the large nonlinear properties of the material influence the optical characteristics of the waveguide. When positioned along a similar D-fiber, the device can be used as a fiber-based, nonlinear coupler controlled by a relatively weak light signal. The same device can be used as an element of a laser and in a number of various other applications.

BACKGROUND

The present invention is concerned with waveguides for optical and nearoptical wavelengths adopted to use nonlinear optical effects ofparticular kinds and of particular materials.

This application claims priority from Swedish Patent Application No.9302634-2, filed Aug. 13, 1993, which is incorporated here by reference.

Optical glass fibers are a cheap wavegulde medium that can be exploitedfor applications in nonlinear optics. Fused quartz, however, has a smallnonlinear coefficient as compared with materials such as semiconductors.This has limited considerably the use of fibers in applications such asoptical switching, because the optical power of the control signal hasto be high in order to cause an appreciable change in the properties ofthe fiber, see "All-optical Waveguide Switching", G. I. Stegeman, E. M.Wright, Optical and Quantum Electronics 22 (1990), pps. 95-122. This ledto the use of discrete components based on LiNbO₃, GaAs and others, see"Integrated Optics in LiNbO₃ : Recent Developments in Devices forTelecommunications", L. Thylen, Journal of Lightwave Tech., 6 (1988)pps. 847-861, "Integrated Optic Devices Based on Nonlinear opticalPolymers", E. V. Tomme, P. P van Daele, R. G. Basts, P. E. Lagasse, IEEEJournal of Quantum Electronics vol. 27, Mar. 1991 and "Physical Conceptsof Materials for Novel Optoelectronic Device Applications II: DevicePhysics and Applications", Proceedings SPIE 1362 (1990), and to thesearch for optical fibers with higher nonlinearity, such as thatachieved with semiconductor doped glasses, see "Optical Nonlinearity andApplicatlons of semiconductor-doped Glass Fiber", D. Cotter, B. J.Ainslie, M. G. Butt, S. T. Davey, R. J. Manning, Proceedings CLEO'91,CTuE7, p. 92, and "Efficient non-linear optical fibers and theirapplication", S. Sudo, Itoh, Optical and Quantum Electronics 22 (1990)pps. 187-212, but that are difficult to fabricate. On the other hand,following the development of optical communications the need exists fora simple fiber-based light controlled coupler. With such a device itwould be possible to derive an adjustable part of the signal transmittedin a fiber to one or several channels in a transmission network. A fiberbased nonlinear optical coupler could also find important applicationsin logical gates and optical transistors, where a weak signal wouldcontrol the switching of a higher power signal.

Recent technological developments led to the fabrication of glass fiberswith non-cylindrical geometry, and in particular the so-called D-shapedfibers, see "Fabrication and Characterization of D-fibers with a Rangeof Accurately Controlled Core/Flat Distances", Electronics Letters 22,March 1986. There, light is guided as in a conventional fiber, but theelectromagnetic field extends all the way to the glass-air interface.This offers a unique opportunity to make light interact through theevanescent field with any material deposited on the flat surface of theD-shaped fiber. Since this interaction takes place longitudinally alongthe fiber, this is a particularly favourable geometry that can beexploited in nonlinear optics.

In the patent U.S. Pat. No. 4,557,551 for Dyott a non-linear opticalfiber coupler is disclosed having two polarization-maintaining opticalfibers of elliptical cross-sections located in parallel at the oppositesides of a central structure. This central structure has a lens-shapedcross-section, that is formed by two oppositely placed arc sections. Thecentral thickness of this middle structure is a few times larger thanthe diameters of the fibers and many times the diameters of the cores ofthe fibers. Also, support structures are used for maintaining the fibersin the desired configurations. The middle structure is a single crystalof an electro-optic material having a non-centro-symmetrical crystalstructure. The materials mentioned are organic type materials beingelectric insulators. The physical effect utilized is based on theelectro-optical effect, the refractive index of the material beingchanged by interaction with the electric field of an incoming lightwave, the pump wave.

In the European patent application EP-A2 0 164 212 for The Board ofTrustees of the Leland Stanford Junior University a fiber opticsaturable absorber is disclosed. An optical fiber has a portion of itscladding removed by polishing a slightly curved fiber along plane. Alight-absorbing substance having non-linear light-absorbingcharacteristics, in particular a dye, is applied to the polished surfaceto make light propagating in the fiber to be absorbed in a controlledway.

In the optic fiber correlator as disclosed in patent U.S. Pat. No.4,927,223 a D-fiber is used being in contact, at its flattened surfacewith a material which is non-centro-symmetrical in order to obtainfrequency doubling. To the opposite fiber ends laser diodes areconnected injecting light of the same wavelengths to the D-fiber. Lightemitted by the layer is collected to find the correlation of the signalsfrom the laser diodes.

In an optical device disclosed in the European patent application EP-A10 254 509 an optical D-fiber may be provided with a layer on the flatsurface, the layer being of a material having a refractive index varyingwith optical intensity. The free surface of the layer is illuminated bycoherent light beams to form a standing wave of a modulated refractiveindex in the layer resulting in a modulated index grating.

SUMMARY

Here we disclose a device that combines the large nonlinearities ofsemiconductor materials with the guiding properties of optical fibers.This device can be used as a fiber-based nonlinear optical coupler thatis controlled by a relatively weak light signal. The same device can beused as a laser, and in a number of other applications.

Thus, generally the invention is concerned with an optical devicecomprising a wave-guide with a core, a layer being arranged, which has auniform thickness and is located in parallel to and at a uniformdistance from the core. The distance is such that the evanescentelectromagnetic field of light propagated along the wave-guide extendsinto the layer. In order to influence the light wave, the material ofthe layer is selected to have non-linear optical properties. Thewave-guide is preferably dimensioned for propagating light of only onewavelength, that is it is monomode.

The material is preferably optically homogeneous or centro-symmetric. Itmay have a refractive index for light of a first kind having aconsidered wavelength and suitable for propagation along the wave-guide,where the value of the refractive index, in particular for wavelengthsclose to the wavelength of the light of the first kind, is dependent, onlight of a second kind which is some manner is arranged to interact withthe material and has a wavelength different from that of the firstlight, that is the refractive index of the material for wavelengthsabout the wavelength of the first light varies significantly, when thereis a change in the second light. In particular, the value of therefractive index may be dependent on the intensity of the light of thesecond kind.

The material is in a preferred embodiment a semiconductor with abandgap. Then as above first light may be propagated along thewave-guide and second light may be arranged to interact with thesemiconductor material in the layer. The wavelength of the second lightcan in that case correspond to an energy value well exceeding thebandgap, so that photons of the second light are absorbed in thematerial creating an electron-hole pair. The wavelength of the firstlight then is selected to have a value corresponding to an energy valuesignificantly below that of the bandgap, so that essentially noelectron-hole pairs are created by absorption of the first light.

For the dimensioning of the device, it can be mentioned that thedistance from the layer to the core of the wave-guide may be smallerthan the diameter of the core to fulfill the condition of the evanescentfield extending into the layer. The thickness of the layer is generallysmall to allow a good adherence to the waveguide and it may inparticular be a small fraction of the diameter of the core of thewave-guide, in particular in the range of 1/8 to 1/80 thereof, mostpreferred in the range of 1/16 to 1/80 thereof.

In one embodiment the wave guide comprises a first optical D-fiber whichas conventional has a curved surface corresponding to a portion of thesurface of a circular cylinder, normally essentially a semi-cylindricalsurface, and a flat surface. The flat surface should then be located inparallel with the core of the D-fiber and at a small distance from thecore and the layer should then be arranged on the flat surface.

In an interferometric device, an optical coupler may be arranged whichas conventional has first and second parallel ports at a first side andfirst and second ports at a second side. The two ports at one side canbe joined by a wave-guide, and a first light source provides signallight to a port at the other side. The layer should then be arranged ata segment of the joining wave-guide in order to control the signallight. Therefore, a second light source may be arranged providingcontrol light and directing it for interaction with the material of thelayer. The second light source is in a first alternative arranged todirect the control light directly to a free surface of the layer. In asecond alternative, the second light source is arranged to insteadprovide the control light to one port at the other side of the coupler.A phase-delaying device is then advantageously arranged in the joiningwave-guide and it delays light derived from the signal light in order toallow the control light to work in an on-off fashion, controlling thesignal light to either one of the ports at the ether side of thecoupler.

The device constructed of a coated D-fiber can for some uses becompleted by a second optical D-fiber also having a substantiallysemi-cylindrical surface and a flat surface, for instance of the samekind as the first one. The flat surface of the second D-fiber is thenarranged against the free surface of the layer, that is the surfacewhich is opposite to the first D-fiber.

Such a double D-fiber device can be used for coupling purposes. Thenthere is a first light source providing signal light, where the lightsource is arranged to provide the signal light to a first end of thefirst D-fiber. A second light source provides control light, so that thecontrol light is input to a first end of the second D-fiber. The firstends of the D-fibers are preferably defined to be those two ends whichpermit the signal and control light to propagate along the cores of theD-fibers in parallel to and in the same direction as each other, but itis also possible to define the first ends so that the signal and controllight propagate in the opposite directions. The second ends of theD-fibers are naturally those which are opposite to the first ends. Thesecond ends are then attached to signal light receiving and/ortransmission means. Alternatively the control light can be provided tothe first end of the first D-fiber, that is the same end as the signallight.

The configuration comprising two juxtaposed D-fibers can becharacterized generally by the feature that the basic wave-guide, inaddition to the core mentioned initially above, comprises another coreextending in parallel to the first mentioned core. The layer should thenbe located at the same uniform distance of the two cores, whereby theevanescent electromagnetic fields of light propagated along the twocores extend into the layer.

In an advantageous, particular embodiment thereof there is a firstD-fiber which has two cores extending in parallel to each other and tothe flat surface and located at the same distance of the flat surfaceand symmetrically in the D-fiber. In another embodiment the devicecomprises a planar structure, where the two cores are arranged in theflat surface a substrate. This surface is covered by a layer of havingsubstantially the same refractive index as the substrate. The surface ofthis layer is in turn coated with a layer of a material havingnon-linear optical properties as above.

These twin-core devices can be used for coupling as the double D-fiber.

For use as a laser the general device as described above may comprise alight source providing pump light. This source is arranged to inject thepump light to a first end of the wave-guide to make it propagate alongthe core into the region where the layer is positioned close to thecore. The pump light is selected to have such a wavelength and intensitythat it will cause stimulated emission of light in the material of thelayer. From a second end of the wave-guide opposite to the first endthereof then the light produced by the stimulated emission can bedirected to some other device.

As to the dimensions of the layer, in this case in particular forconfinement of the lasering region, the layer has a width, that is adimension in a transverse direction in relation to the longitudinaldirection of the wave-guide core and in parallel with the surface of thewave-guide, which corresponds to only a few times the diameter of thecore, in particular three core diameters at most, and at least one corediameter. Then the layer may be located on a flat surface of thewave-guide or it may be located in a groove in the material of thewave-guide, where the groove then extends in parallel to the wave-guidecore. The interferometric device can also be described as comprising acoupling means having a first and a second pair of optical communicationports in which optical pulse signals received at a port of one pair arecoupled substantially equally into the each port of the other pair. Itfurther comprises an optical waveguide optically coupling together thesecond pair of ports. The optical waveguide includes a portion wherelight propagating in the waveguide will sense a non-linear refractiveindex, where this portion comprises a waveguide having a core and apiece of material, which typically is the "layer" mentioned above, beinglocated at such a distance from the core such that the evanescentelectromagnetic field of light propagated along this waveguide and itscore extends into the material. This material should then exhibitnon-linear optical characteristics. The piece of material or "layer" canthen have a free surface, arranged so that a light source providescontrol light to the free surface for interaction with the material.Alternatively, a light source is arranged to provide control light toone port of the first pair.

The optical device for use for coupling purposes can be generallydescribed as comprising a wave-guide with two cores for guiding lightwaves along each one of the cores, where a piece of material located atsuch distances from the cores, that the evanescent electromagnetic fieldof light propagated along each wave-guide core extends into the piece ofmaterial. As above, the material thereof should have non-linear opticalproperties.

An optically controlled coupler system, switching or modulating system,including such a two-core device, can be described as comprisinggenerally a coupling means having a first and a second pair of opticalcommunication ports. It further comprises a first light source providingsignal light to a first one of the first pair of ports and a secondlight source providing control light to the first one or a second one ofthe first pair of ports. The ports of the second pair are each one asabove attached to separate signal light receiving and/or transmissionmeans. The coupling means comprises a wave-guide with two cores forguiding light waves along each one of the cores and a piece of material,the above "layer", located at such distances from the cores, that theevanescent electromagnetic field of light propagated along eachwave-guide core extends into the piece of material. Like above, thematerial should have nonlinear optical properties.

An optically pumped laser structure as set out above comprises thengenerally a wave-guide with a core. It further comprises a piece ofmaterial or layer located at such a distance from the core, that theevanescent electromagnetic field of light propagated along thewave-guide extends into the piece of material or layer. The materialshould then have non-linear optical properties. One end of the waveguideis adapted to receive pump light from a light source and the other endis adapted to issue light obtained by stimulated emission in the pieceof material.

Also in this case, the waveguide may have a second core, such that lightwaves are able to propagate along each one of the two cores. The pieceof material is then located at such distances from the cores, that theevanescent electromagnetic field of light propagated along each one ofthe wave-guide cores extends into the piece of material.

The piece of material or layer may in the laser structure have a freesurface being adapted to receive pump light from a light source.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described as a number of non-limitingembodiments, with reference to the accompanying drawings in which

FIG. 1 shows a partially coated D-fiber,

FIG. 2 shows the fiber of FIG. 1 as employed in an interferometricdevice,

FIG. 3a shows a double D-fiber structure with a film therebetween,

FIG. 3b schematically shows the structure of FIG. 3a as employed in anoptic coupler,

FIG. 4a shows a coated D-fiber structure of the twin-core type,

FIG. 4b shows a planar wave guide structure with a layer of a non-linearoptic material,

FIG. 5a shows a partially coated D-fiber for use as a laser,

FIGS. 5b and 5c show alternative coating arrangements which may besuitable for use in lasers,

FIG. 6 schematically shows a laser arrangement utilizing a coatedD-fiber,

FIG. 7 schematically shows a laser arrangement utilizing a structurehaving two wave guide cores.

DETAILED DESCRIPTION

Single D-fiber

In FOG. 1 a device is illustrated comprising a D-shaped fiber 1 having acore 3 and a cladding 5 and coated on its flat surface over some lengthof the fiber with a thin layer 7 of a material that is highly nonlinear,such as a semiconductor. The length of the coating in the longitudinaldirection of the fiber may as illustrated correspond to about theexterior diameter of the D-fiber and the coating may cover whole theflat surface of the fiber in the transverse direction. The length mayhowever be adapted to the particular field of use and the materials usedto obtain the desired effects to be discussed below. The D-fiber 1 mayhave dimensions matching ordinary single-mode communication fiber, e.g.have an outer cladding diameter of approximately 125 μm and a corediameter of 8-10 μm, and should also ideally be single mode at thewavelength of interest.

The core-flat surface distance d in the D-shaped fiber 1 should be suchthat the evanescent field of the light propagating through the fiber ican interact with the film 7, this meaning that the core-flat distance dis only a small portion of the core diameter. The D-fiber to be usedshould then in practical cases have a core-air distance d of a fewmicrometers or less, that is this distance d of the core 3 to the flatsurface of the fiber 1 can be for instance of the magnitude of order 2μm.

The thickness of the coating film 7 is uniform and is normally muchsmaller than the diameter of the core. It may be typically 1 μm or less,for instance 0.1-0.5 μm. A thicker film cannot be used since it will notadhere permanently to the flat surface of the D-fiber due to differentdilatation coefficients.

The coating material in the layer 7 can be GaAs, InP, InGaAsP, and otherIII-V semiconductors, silicon and germanium based materials, such asamorphous Si, amorphous Go, silicon carbide, porous Si, etc., II-VIsemiconductors such as CdTe, CdS, case and composites, and materialssuch as LiF, NaF, NaCl and other halogen alkalides. A preferred materialfor the film of many devices may be amorphous Si. The material can bedeposited by some ordinary method such as some CVD technique and it isgenerally not necessary to grow the material in situ. It can also befurther processed by ion or e-beam bombardment or UV irradiation. Thefabrication of the device might also include an annealing process wherethe deposited material is heated and then cooled to the ambienttemperature in order to alter the properties of the material, such asthe recombination time of the charge carriers thereof. Structurescomprising multiple thin films such as a single and multiple quantumwell structures of III-V or other semiconductors can also be depositedon the flat surface of the fiber i and act as the highly nonlinearmedium. The material deposited can either be amorphous or crystalline.Other materials of interest, such as films of rare-earth metals can beused. In the following we will refer to the film 7 as made of asemiconductor material, but all other materials described above are inthe spirit of the invention. We will refer to the device as aSemiconductor Coated Fiber (SCF).

Physical Process

When a semiconductor, metallic or dielectric material 7 is deposited onthe flat surface of a D-shaped fiber like 1, the wave propagating in thelongitudinal direction of the fiber i can have its field extending intothe material 7. In this case, the transverse mode of the propagatinglight depends on the real and imaginary parts of the linear andnonlinear susceptibilities of the material of the film 7. Together withthe parameters of the fiber, such as the refractive indices of the core3 and cladding 5, the radius of the core 3, and the distance d core-flatsurface, the properties of the material determine the effectiverefractive index experienced by the light propagating in the fiber, thechromatic dispersion, absorption, gain, polarization properties, andnonlinear effects such as phase modulation and frequency conversion forthe light.

Signal and Control Light

In a number of applications of the devices disclosed herein light of twodifferent wavelengths is used, referred to as "signal" light and"control" light (corresponding to the terms "first light" and "secondlight"). Generally (but not necessarily) the signal light has a photonenergy close to or below the energy which is necessary to promoteelectrons from the valence (low mobility) to the conduction (highermobility) band, i.e., the deposited material is "transparent" to thesignal light. On the other hand, for the control light, the photonenergy exceeds the energy band gap of the material. As an example, lightat a wavelength 0.85 μm can be used as control light for a film ofInGaAs (bandgap for instance typically 1.3-1.55 μm). In this case, theabsorption of the control light photons creates mobile electrons in thematerial, temporarily affecting its properties. It can strongly alterthe refractive index, and also shift the band gap through the Starkeffect. Even the creation of mobile electrons through heat can induce achange in the refractive index.

Thus, generally materials are used, the optical properties of which fora particular wavelength, i.e. that of the signal light, are changed whenthe materials interact with the control light in some way, for exampleas above by absorption of photons creating electron-hole pairs, byabsorption creating heat in the material increasing its temperature. Thematerials preferred are optically homogeneous, for example amorphous,resulting in structures which can be manufactured easily and non-costly.

Interferometer

A change in the refractive index of the coating material can beexploited amongst others, for switching, for modulation, and formode-locking lasers. It can be advantageous to make use of aninterferometric arrangement such as that disclosed in the patent U.S.Pat. No. 4,973,122 and also in U.S. Pat. No. 4,962,987. There an opticalcross-coupled interferometer is disclosed where the control light is fedto one of the ports of the coupler and two parallel ports are joined bya loop. Incoming light is split in two parts (generally of the samepower), which suffer different phase shifts depending on the presence ofthe control light. When they are recombined, even a small influence ofthe control light can greatly affect the amplitude of the emergingsignal light.

One example of arrangements that can be used is illustrated in FIG. 2,where the so-called nonlinear loop mirror configuration is used. Acoupler 9 has two parallel ports A and D at one side and two parallelports B and C at the other side. The signal light P_(s) is coupled intothe port A of the coupler 9 that divides the signal light P_(s) into twocomponents of approximately equal power, to the ports B and C at theother side of the coupler. A device 11 of the kind illustrated in FIG. 1is at one end connected to one of these ports like B. The other end ofthe device 11 is then joined to the other parallel port C at the sameside. In this way, half of the signal light leaving the coupler 9travels through first port B, through the device 11 and then to port C.The other half travels first through port C, and then through the device11 and back to port B. Both light components then recombine at thecoupler 9. A set of fiber strainers 13 such those designated as ManualPolarisation Controllers, manufactured by the company BT & D, can beprovided along one of the paths, e.g. as illustrated in the path joiningthe device 11 with port C.

In the absence of control light the fiber strainers 13 can be adjustedso that all signal light recombines at the coupler 9 and returns throughthe input port A. In this state, no signal light exits through port D,that is the port which is parallel to port A and at the same side of thecoupler. In the presence of control light P_(c), however, e.g. asobtained from a light source comprising a laser 13 and a lens system 15and illuminating the surface of the film of the device 11, therefractive index of the deposited material in the coating film in thedevice 11 is altered. For a given control light power, the relativephase shift introduced between the signal light components can lead toall signal light recombining at the coupler 9 being directed to port D.The exact position of the device 11 in the loop joining ports B and Ccan be important, particularly if the control light is continuous andnot pulsed.

The control light may also be injected by port A and D producing thesame change of refractive index of the coating in the device 11.

Nonlinear Coupler--Modulator

The devices described above make use of a single SCF. Two D-shapedfibers 1 can also be placed with the flat surfaces in a back-to-backarrangement, with the semiconductor film 7 lying between the two cores3, as shown in FIG. 3a. We call this the double structure SCF. In orderto position the fibers as illustrated, with the two cures in parallelwith each other and at a minimum distance, two fixtures can be used,each one with one fiber. The fixtures can be made of a softer materialthan glass, such as a plastics material or soft metal. One possible wayto make the fixtures is to use a D-fiber as a mould for a liquid phasesolution of the fixture. The D-fiber should be kept straight. Aftersolidifying the solution, the fixture is ready to accommodate one of theD-fibers used in the double structure SCF. Once the fibers arepositioned as illustrated in FIG. 3a with the cores 3 in parallel, thefixtures can be glued together.

The nonlinearity of the semiconductor film 7 can in this case beexploited by sending control light P_(c) of appropriate wavelengththrough an input control port D of the device as illustrated in FIG. 3aused as a controlled coupler 17, c.f. FIG. 3b, having ports A and D atone end thereof and ports B and C at the other end. Signal light P_(s)entering port A in parallel to the aforementioned control light P_(c)can be affected by the control light P_(c) entering port A. In theabsence of the control light P_(c), the signal light P_(s) leaves thedevice 17 at either port B or C as in a conventional coupler, see"Optical Waveguide Theory", A. W. Snyder, J. D. Love, Chapman and Hall,London 1983, pps. 387-399, 568-574. In the presence of control lightP_(c), the fraction of the light leaving ports B and C can becontrolled. Even a weak signal P_(c) can substantially modify thetransfer function of the coupler, and therefore a weak signal P_(c) cancontrol a stronger signal P_(c) as in a transistor.

The nonlinear coupler described above can also be used as a high speedoptical modulator. A continuous wave signal (signal light) P_(s) coupledinto port A of the device can be switched on and off in ports B and C bya control light pulse P_(c) coupled into port D. When the modulationdepth is sufficiently large, the device operates as a switch. Since thedevice is relatively insensitive to the wavelength of the control lightP_(c), even heavily chirped light pulses can be used to control thespectrally pure signal P_(s) coupled into port A. The high speed of thedevice is based on the fact that the free-carriers photoinduced in thesemiconductor in the film 7 in the device 17, by the absorption of thecontrol light pulse can relax rapidly, so that bit rates of severalGbits/s should be achieved. The optical signal that leaves the devicethrough ports B and C can therefore have a narrow bandwidth and highspeed modulation.

Integrated Twin-Core Structures

Although the specific geometry described above is convenient for manyapplications, other arrangements can be used in the same spirit. Forexample, a twin-core D-shaped fiber can have the couplingcharacteristics controlled by the activation of a semiconductor film, asillustrated in FIG. 4a. The D-fiber 19 has two cores 3', 3", which aredisposed in parallel symmetrically and centrally, close to the centerline of the circular-cylindrical outer surface of the fiber 19 and asabove close to the flat surface. Here also, the evanescent field of thecontrol light propagating in one of the fiber cores 3', 3" penetratesthe semiconductor film 7 coated on the flat surface, altering itsproperties, and thus the propagation characteristics of signal lighttravelling in the other core. The twin-core structure of FIG. 4a can beused like the double D-fiber of FIG. 3a, e.g. in the coupler asillustrated in FIG. 3b.

Planar Structures

Even discrete components based on a glass substrate with buriedwaveguides can be used for controlled coupling, switching andmodulation. As is illustrated in FIG. 4b, a pair of waveguides 21 can befabricated on a glass substrate 23 by ion implantation or thermalindiffusion. Further evaporation of a layer 25 of silicon oxidesSiO/SiO₂ might be desirable, leaving the waveguides 21 immersed in theglass matrix. A film 7 of semiconductor material is then deposited onthe top surface of the buried waveguides 21, so that the evanescentfield of the light in the waveguides 21 extends into the semiconductormaterial 7. Switching from one into various channels should be possible,and the fraction of the signal switched can be controlled by light,travelling along a waveguide or incident on the device from the outside(e.g., perpendicularly). The buried wave-guide structure of FIG. 4b canbe used like the double D-fiber of FIG. 3a.

A similar idea has been suggested by Kawachi in "Silica Waveguides onSilicon and their Application to Integrated-Optic Components", Opticaland Quantum Electronics 22 (1990), pps. 391-416, however in that casethe waveguide was fabricated on top a semiconductor material, e.g.silicon.

Laser

The devices comprising a SCF described so far make use of a controllight input that effectively alters the attenuation and the refractiveindex of the material. Semiconductors, on the other hand, are widelyused as laser materials. Generally, laser pumping is electrical, butlight has also been used to pump laser structures. The pump light shouldhave enough photon energy to be absorbed by the film, creating an excessof electrons in the excited state and an excess of holes in the lowerenergy band. Upon radiative recombination, the semiconductor willluminesce. Provided the pump light is sufficiently intense it will causestimulated emission and laser action in the semiconductor film. The highrefractive index of the semiconductor material favours guiding of lightin the plane of the semiconductor film. Gain guiding is often sufficientto ensure that the device operates above laser threshold. Since the pumpfield overlaps the semiconductor film only in the region neighboring thecore of the fiber, the width of the film where laser operation is takingplace, is limited to less than approximately 10 μm. A structure similarto that depicted in FIG. 1 can be used as is illustrated in FIG. 5a.

In order to further confine the field to the pumped region adjacent tothe core of the fiber, the unpumped region of the semiconductor may beremoved by lithographic processes, as illustrated in FIG. 5b. On theflat surface of the D-fiber 1 there will thus only be a narrow band orstrip 7' of a semiconductor material extending centrally on the surface,in parallel to the fiber core 3. Alternatively, the D-shaped fiber canhave its cross sectional profile altered as shown in FIG. 5c. On theflat surface of the D-fiber 1' there is a shallow groove 27 close to andin parallel to the fiber core 3 and the semiconductor film 7" isdeposited in the groove 27. This will ensure that only the narrow strip7' or 7" of semiconductor material guides light.

The structures of FIGS. 5b and 5c will be able to provide laser actioneven in the absence of end mirrors, provided that the gain issufficient. End mirrors may otherwise be provided by the use of coatedor uncoated fiber end surfaces or by Bragg reflection. The active lengthof the laser can be as long as several centimeters corresponding to thelongitudinal length of the coating 7 (FIG. 5a). Typical materials thatcan be employed would be those which have a high quantum efficiency forluminescence. III-V semiconductors such as GaAs deposited by MOVCD orMBE techniques, for example, can be used, as well as ternary compounds.The wavelength for laser operation will be those associated with thematerial bandgap (e.g. 0.85 μm for GaAs). Other materials of interestwill be halogen alkalide films with colour centers, such as the f-centerof LiF or LiI. Such materials have a number of luminescence peaks in thevisible and near infrared. The light generated in such a laser is guidedalong the film, but the near presence of the core of the fiber can alsolead to light guidance in the core.

A laser arrangement is schematically illustrated in FIG. 6. Pump light29 from a suitable source 31 focused by a lens system as illustrated at33 is injected to one end of the D-fiber 1. With a proper choice ofcomponents and parameters laser light 35 is emitted from the other endof the D-fiber and it can be parallelized by a lens system asillustrated at 37.

The geometry with two parallel wave guide cores as illustrated in FIGS.3a and 3b and also in FIGS. 4a and 4b can also be used as a laserdevice, where the laser output would be collected along the output portsB and C, cf. FIG. 7, with the pump light injected along port A. When thepump light P_(p) is absorbed by the semiconductor film 7 in the coupler17", it may as above by a proper choice of parameters and materials giverise to an excess of electrons in the excited state band and an excessof holes in the lower energy band. Upon radiative recombination, thesemiconductor material will luminesce.

Although the aforementioned arrangements are convenient from a practicaldevice viewpoint, other pumping geometries can be considered, such asexternally pumping a single-structure semiconductor coated fiber of thekind described with reference to FIGS. 1 and 2.

What is claimed:
 1. An optical device, comprising:a waveguide having afirst substantially cylindrical core; and a layer having a uniformthickness and located parallel to and at a uniform distance from thefirst core, wherein the distance is such that an evanescentelectromagnetic field of light propagated along the waveguide extendsinto the layer, and the layer is of a material having nonlinear opticalproperties.
 2. The optical device of claim 1, wherein the waveguide ismonomode, constructed for propagating light of only one wavelength. 3.The optical device of claim 1, wherein the material is opticallyhomogenous or centro-symmetric.
 4. The optical device of claim 1,wherein the material has a refractive index for first light of awavelength propagated along the waveguide, a value of the refractiveindex depending on second light interacting with the material and havinga different wavelength.
 5. The optical device of claim 1, wherein thematerial has a refractive index for first light of a wavelengthpropagated along the waveguide, a value of the refractive indexdepending on an intensity of second light interacting with the material.6. The optical device of claim 1, wherein the material is asemiconductor having a bandgap, first light is propagated along thewaveguide, second light is arranged to interact with the semiconductormaterial in the layer, a wavelength of the second light corresponds toan energy value well exceeding the bandgap so that photons of the secondlight are absorbed in the material creating electron-hole pairs, and awavelength of the first light has a value corresponding to an energyvalue significantly below that of the bandgap so that substantially noelectron-hole pairs are created by absorption of the first light.
 7. Theoptical device of claim 1, wherein the distance from the layer to thefirst core of the waveguide is smaller than a diameter of the firstcore.
 8. The optical device of claim 1, wherein a thickness of the layeris a small fraction of a diameter of the first core of the waveguide,the fraction being in a range of 1/8 to 1/80, most preferably in a rangeof 1/16 to 1/80.
 9. The optical device of claim 1, wherein the waveguidecomprises a first optical D-fiber having a substantiallysemi-cylindrical shape and a flat surface, the flat surface is locatedparallel to and at a small distance from a core of the D-fiber, and thelayer is arranged on the flat surface.
 10. The optical device of claim1, further comprising an optical coupler having first and secondparallel ports at a first side and first and second ports at a secondside, wherein the first and second parallel ports at the first side arejoined by a joining waveguide, a first light source is arranged toprovide signal light to one of the first and second ports at the secondside, and the layer is arranged at a segment of the joining waveguide.11. The optical device of claim 10, further comprising a second lightsource arranged to provide and direct control light for interaction withthe material of the layer.
 12. The optical device of claim 11, whereinthe second light source is arranged to direct the control light directlyto a free surface of the layer.
 13. The optical device of claim 11,wherein the second light source is arranged to provide the control lightto one of the first and second ports at the second side of the opticalcoupler.
 14. The optical device of claim 10, further comprising aphase-delaying device arranged in the joining waveguide, wherein thephase-delaying device delays light derived from the signal light. 15.The optical device of claim 9, further comprising a second opticalD-fiber having a substantially semi-cylindrical shape and a flatsurface, wherein the flat surface is arranged against a surface of thelayer that is opposite to the first D-fiber.
 16. The optical device ofclaim 15, further comprising:a first light source arranged to providesignal light to a first end of the first D-fiber; a second light sourcearranged to provide control light to a first end of the second D-fiber;and second ends of the first and second D-fibers opposite the respectivefirst ends and attached to means for receiving and/or transmittingsignal light.
 17. The optical device of claim 15, furthercomprising:first ends of the first and second D-fibers; a first lightsource arranged to provide signal light to the first end of the firstD-fiber; a second light source arranged to provide control light to thefirst end of the first D-fiber; and second ends of the first and secondD-fibers opposite to the first ends and attached to means for receivingand/or transmitting signal light.
 18. The optical device of claim 9,wherein the first D-fiber has two cores extending parallel to each otherand to the flat surface, and the two cores are located symmetrically inthe first D-fiber at the same distance from the flat surface.
 19. Theoptical device of claim 1, wherein the waveguide further comprises asecond core extending parallel to the first core, and the layer islocated at the same uniform distance from the first and second coressuch that evanescent electromagnetic fields of light propagated alongthe first and second cores extend into the layer.
 20. The optical deviceof claim 19, wherein the device has a planar structure, the first andsecond cores are arranged in a flat surface of a substrate, the flatsurface is covered by a layer which has a refractive index that issubstantially the same as a refractive index of the substrate and whichis coated with a material having nonlinear optical properties.
 21. Theoptical device of claim 19, further comprising:a first light sourceproviding signal light, wherein the first light source is arranged toinject the signal light into the waveguide to make the signal lightpropagate along a first core of the waveguide from a first end of thefirst core; a second light source providing control light, wherein thesecond light source is arranged to inject the control light into thewaveguide to make the control light propagate along a second core of thewaveguide from a first end of the second core; and second ends of thefirst and second cores opposite the respective first ends and attachedto respective means for receiving and/or transmitting signal light,wherein each receiving and/or transmitting means receives lightpropagating along its respective one of the first and second cores. 22.The optical device of claim 19, further comprising:a first light sourceproviding signal light, wherein the first light source is arranged toinject the signal light into the wavegulde to make the signal lightpropagate only along a first core of the waveguide from a first endthereof; a second light source providing control light, wherein thesecond light source is arranged to inject the control light into thewaveguide to make the control light propagate along the first core ofthe waveguide from the first end; and a second end of the waveguideopposite the first end and attached to two means for receiving and/ortransmitting signal light, wherein the two receiving and/or transmittingmeans receive light propagating along a corresponding one of the firstand second cores.
 23. The optical device of claim 1, further comprisinga light source providing pump light, wherein the light source isarranged to inject the pump light in one end of the waveguide to makethe pump light propagate along the first core into a region where thelayer is positioned close to the first core, the pump light has awavelength and an intensity that stimulates emission of light in thematerial of the layer, and one end of the waveguide is arranged tooutput the light produced by the stimulated emission.
 24. The opticaldevice of claim 1, wherein the layer has a width that is at leastsubstantially equal to a diameter of the first core and at mostsubstantially three times the diameter of the first core.
 25. Theoptical device of claim 19, wherein the layer is located in a groove inthe material of the waveguide, and the groove extends parallel to thefirst and second cores.
 26. An interferometric device, comprising:means,having a first pair and a second pair of optical communication ports,for coupling optical pulse signals received at a port of one pair of thefirst and second pairs substantially equally into each port of anotherpair of the first and second pairs; and an optical waveguide opticallycoupling together the second pair of ports, wherein the opticalwaveguide includes a portion where light propagating in the waveguidesenses a nonlinear refractive index; the portion comprises a waveguidehaving a core and a piece of material located at a distance from thecore such that an evanescent electromagnetic field of light propagatedalong the waveguide and core extends into the material; and the materialhas nonlinear optical characteristics.
 27. The interferometric device ofclaim 26, wherein the material has a free surface, and theinterferometric device further comprises a light source providingcontrol light, the light source arranged to direct the control light tothe free surface for interaction with the material.
 28. Theinterferometric device of claim 26, wherein a light source is arrangedto provide control light to one port of the first pair.
 29. An opticaldevice, comprising:a waveguide having two substantially cylindricalcores for guiding light waves along each core; and a piece of materialhaving nonlinear optical properties, wherein the piece of material islocated at distances from the cores such that an evanescentelectromagnetic field of light propagated along each core extends intothe piece of material.
 30. The optical device of claim 29, wherein thewaveguide comprises two optical D-fibers placed on each side of thepiece of material.
 31. The optical device of claim 29, wherein thewaveguide comprises an optical D-fiber having two cores.
 32. The opticaldevice of claim 29, wherein the waveguide comprises a planar structurehaving two cores extending in parallel in a first part, and the piece ofmaterial is located on a surface of the first part.
 33. The opticaldevice of claim 32, wherein the first part includes a substrate havingthe two cores arranged in a surface thereof, and the surface is coveredby a layer of a material that is optically similar to a material of thesubstrate.
 34. An optically controlled system for carrying out at leastone of the functions of coupling, switching, and modulating, the systemcomprising:coupling means, comprising a first pair and a second pair ofoptical communication ports, a waveguide having two cores for guidinglight waves along each core, and a piece of material having nonlinearoptical properties and located at distances from the cores such that anevanescent electromagnetic field of light propagated along each coreextends into the piece of material; a first light source for providingsignal light to a first one of the first pair of ports; and a secondlight source for providing control light to the first one or a secondone of the first pair of ports, wherein the ports of the second pair areeach attached to a respective means for receiving and/or transmittingsignal light.
 35. An optically pumped laser structure, comprising:awaveguide having a core; and a piece of material having nonlinearoptical properties and located at a distance from the core such that anevanescent electromagnetic field of light propagated along the waveguideextends into the piece of material; wherein one end of the waveguide isadapted to receive pump light from a light source, and at least one endof the waveguide is adapted to issue light obtained by stimulatedemission in the piece of material.
 36. The laser structure of claim 35,wherein the waveguide comprises a D-fiber, and the piece of material islocated on a flat surface of the D-fiber.
 37. The laser structure ofclaim 35, wherein the waveguide comprises a D-fiber, and the piece ofmaterial is located in a groove in a flat surface of the D-fiber.
 38. Anoptically pumped laser structure, comprising:a waveguide having a firstcore and a second core, wherein light waves are able to propagate alongeach core; and a piece of material having nonlinear optical propertiesand located at distances from the first and second cores such thatevanescent electromagnetic fields of light propagated along the coresextend into the piece of material; wherein one end of the waveguide isadapted to receive, at one end of the first core located at the one endof the waveguide, pump light from a light source, and another end of thewaveguide is adapted to issue, at ends of the first and second coreslocated at the other end of the waveguide, light obtained by stimulatedemission in the piece of material.
 39. An optically pumped laserstructure, comprising:a waveguide having a core; and a piece of materialhaving nonlinear properties and located at a distance from the core suchthat an evanescent electromagnetic field of light propagated along thewaveguide extends into the piece of material, wherein the piece ofmaterial has a free surface adapted to receive pump light from a lightsource, and at least one end of the waveguide is adapted to issue lightobtained by stimulated emission in the piece of material.